One of the major challenges for the next generation of wireless communication systems is considerable traffic-growth. Current systems such as Long Term Evolution (LTE) already provide very high peak data rates per user both in the uplink and the downlink. Therefore, it seems reasonable to consider that such a predicted traffic growth accounts for higher aggregate data rate, i.e. higher spectral efficiency for many simultaneous users, rather than increasing the per-user spectral efficiency.
Code division multiple access (CDMA) is one of the most spectrally efficient schemes when considering several users. In CDMA type systems, several code-words related to different users are transmitted concurrently over the same resource (i.e. summed) after being assigned a user-specific signature and spreading. Generally, in a generic CDMA transmission, the number of signature sequences is equal to the number of chips.
An overloaded situation is where there are more signature sequences than chips. In this case, a set of orthogonal signatures does not exist and interference in the transmitter is inherent. Using more signatures than available chips permits scheduling more users compared to orthogonal transmission and fulfills the massive connectivity requirement necessary for the next generation of communication systems.
When it comes to detection, the optimum Maximum A Posteriori (MAP) multi user detection can be done by using an algorithm which performs an exhaustive search over a large set of possible transmitted signals. Such a demodulator is typically too complex for practical use, as it has to go through all possible sequences of x∈K. In other words, this demodulator evaluates |K| signal alternatives to find the solution for each chip. Therefore, the total number of alternatives (or metrics) for complete demodulations are N||K. However, it has been demonstrated that the complexity of the MAP demodulator could be reduced by using specially designed sparse signatures which allow to employ simpler receiver algorithms.
Such sparse signatures contain only few non-zero elements and are referred to as Low-Density Spreading (LDS) signatures or low density signature matrices. So far, Low Density Parity Check (LDPC) code matrices were used as LDS signature matrices. The constraints applied in the generation of LDPC matrices imply that the corresponding graph representation contains cycles. This characteristic is key for enabling the use of iterative decoding algorithms at the receiver.
In the design of signatures so far it was anticipated that matrices designed according to the conventional LDPC structure will perform adequately. These matrices are low density as previously discussed and have cycles in their graph representation.
Results obtained show that the LDS iterative detector tailored to signatures of the LDPC type can achieve a robust near single-user performance with overloading factors of up to 2 when using Binary Phase Shift Keying (BPSK). Using more complex modulations, such as Quadrature Phase Shift Keying (QPSK), the aforementioned detector approaches the single-user performance with a larger gap for overloading factors up to or more than 2.
It would be advantageous to provide single-user performance with a smaller gap for overloading namely when QPSK and overloading factors higher than 2 are used. It would also be advantageous to increase the aggregate spectral efficiency in multiple access transmission schemes of LDS type while reducing receiver complexity. Furthermore, it would be advantageous to jointly optimize the LDS signature and LDS transceiver to provide reasonable performance even when higher overloading factors are employed.